1. Field of the Invention
This invention relates to semiconductor transducers and in particular, to a hermetically sealed ultra high temperature silicon carbide pressure transducer and method for fabricating the same.
2. Description of Related Art
Semiconductor pressure transducers are employed in the measurement of pressure in numerous types of applications and environments. Many pressure transducers employ a relatively thin diaphragm fabricated from semiconductor material such as silicon. Upon the diaphragm is deposited or diffused a piezoresistive strain gage configuration, such as a bridge circuit, whereby the resistors associated with the bridge exhibit a change in resistance according to a deflection in the diaphragm. By monitoring the change in resistance, one obtains an output voltage indicative of the applied pressure or force.
Pressure transducers capable of operating at high temperatures of at least 600.degree. C. and having small physical dimensions are highly desired in various applications. High temperatures can be associated with high pressure environments. Contact areas of transducers generally are formed of metal which can oxidize. Thus, the nature of the operating environment results in prolonged exposure of both the metalization of the transducers and the lead attachments to high temperature in an oxidizing atmosphere.
Formerly, monolithic sensors fabricated integrally with a silicon diaphragm and operating temperatures of about 350.degree. F. would deteriorate due to thermally generated carriers which served to short circuit the sensors to the substrate, because isolation of the silicon stress sensors from the force collecting structure by PN junctions in these monolithic devices deteriorated as a function of temperature. T avoid this problem, dielectrically isolated sensors were fabricated. For example, see U.S. Pat. No. 3,800,264 to A. D. Kurtz et al., entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS. See also U.S. Pat. No. 3,930,823 to Kurtz et al., entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS, both assigned to Kulite Semiconductor Products, Inc., the assignee herein. These devices provide dielectric isolation of the sensor from the diaphragm and operate at temperatures in excess of 500.degree. C. When temperatures reach above 600.degree. C., however, the silicon sensing network and the silicon force collector undergo significant plastic deformation rendering the device useless as a pressure transducer.
To overcome this problem, high temperature transducers employing silicon carbide have been fabricated. For example, see U.S. Pat. No. 5,165,283 to A. D. Kurtz et al., entitled HIGH TEMPERATURE TRANSDUCERS AND METHODS OF FABRICATING SAME EMPLOYING SILICON CARBIDE, assigned to the assignee herein. A heteroepitaxial growth process for growing alpha silicon carbide in the fabrication of pressure transducers having a diaphragm fabricated from one type of silicon carbide and a sensor fabricated from another type of silicon carbide is described in the patent. These transducers are capable of operating at extremely high temperatures in excess of 600.degree. C.
Problems in employing semiconductor pressure transducers at elevated temperatures includes the deterioration of the stress sensing network due to the high temperature, and oxidizing atmospheres leading to corrosion due to the presence of oxygen in the environment an the reactive metal contact areas. Methods enabling the transducer to remain operational in these corrosive and high temperature environments involve exposing the backside of the transducer to ambient pressure while hermetically sealing the stress sensing network on the active portion of the diaphragm. Such a method is described in co-pending U.S. application Ser. No. 08/711,078 filed Sep. 9,1996, which is a continuation of Ser. No. 08458,405, filed Jun. 2, 1995, entitled HEMETICALLY SEALED TRANSDUCERS AND METHODS FOR PRODUCING THE SAME by A. D. Kurtz and assigned to Kulite Semiconductor Products, the assignee herein. The hermetic seal is formed by glass frit bonding a cover member having a central aperture to a transducer wafer. While in a vacuum environment the aperture is sealed, thereby maintaining a vacuum between the transducer element and the cover. A portion of the electrical contact is left exposed to enable subsequent wire bonding thereto.
One of the major problems in employing silicon carbide in pressure transducers at elevated temperatures and in oxidizing atmospheres is the deterioration of the contact areas. Contact areas are generally formed of materials such as titanium and tantalum silicide which are overcoated platinum. Fine gold wire bonds are then attached metalized contacts by ultrasonic ball bonding. The opposite end of the gold wire is frequently bonded to a gold-plated nickel or KOVAR pin in the header assembly. An example of a header assembly is found in. U.S. Pat. No. 4,764,747 to Kurtz et al., assigned to the assignee herein.
Although the technique of forming contact areas of titanium and tantalum silicide overcoated with platinum has bee useful for short terms, under prolonged exposure to harsh environments, the metal layers are attacked and oxidized. Moreover, the gold from the ultrasonic bond on the silicon carbide contact area may diffuse through the contact metalization into the underlying silicon carbide thereby resulting in device failure. In addition, the gold bond to the gold-plated pin also deteriorates due to the diffusion of the gold into the pin and the subsequent oxidation thereof. Techniques for hermetically sealing the sensing network from hostile environments have heretofore limited the size of the transducers produced, since additional lateral space was required in accommodating a sealing cover structure.
There remains a need for ultra high temperature silicon carbide pressure transducers and method of making the same which prevent the deterioration of the contact area when exposed to high temperatures of 600.degree. C. or grater and to oxidizing atmospheres which extends the useful life and optimizes the yield of transducers produced and eliminates the need for gold wires ultrasonically bonded to the contact areas and to header pins.
In addition affixing a silicon-carbide sensor diaphragm directly to a header or sealing it to a glass structure before affixing it to a header gives rise another problem because of the large mismatch of thermal expansion coefficients between the silicon carbide sensor diaphragm and either the header material or the glass structure. This causes thermally induced strain in the sensor and contributed to a measurement error.